Electronics > Repair

Fluke/Philips PM3384a combiscope digital trace faults

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Guineafowl21:
Hi all,

In analogue mode, the CAL signal displays fine on all four channels.

In digital mode, channels 1 and 3 display the trace off the top of the screen, while 2 and 4 display it off the bottom. Occasionally, the trace wobbles into view, and is distorted, lots of roll-off etc.

The self tests all pass, but the autocal gets stuck after 3 minutes or so.

I have the service manual, but can’t make head nor tail of the DAC repair tool function described near the end.

Is there a firmware update?

Anyone else had this fault? I found a thread on this forum describing something similar, but can’t find it again and anyway it didn’t really conclude. I gather a user called HighVoltage may be able to help.
Cheers for now.

Guineafowl21:
Quick update: For whatever reason, all the digital traces are now below the screen. Also, the autocal does complete, but takes about 10 minutes (should be 4) and ends with a message to repeat the process. Status display says ‘calibration error 101’.

Guineafowl21:
I’ve traced the signal from the front end, and it makes it onto the digital board (using square CAL wave as test signal).

It goes through a buffer/splitter OK, then through the MASPU (main aquisition signal processing unit).

From there, it reaches an upright, brown-coated hybrid chip pins 1 and 2. The output is on pin 3. The wave is still there, but looks distorted. I’m not sure if this is my measurement technique, though.

This then goes to a heat-sinked flash ADC. No data emerges, but I’m not experienced in this digital stuff. The power supply appears to be GND and -5V, so I expect the data to be a fast 0 to -5V signal.

Perhaps someone might be able to suggest what signal to be expected at each stage? Should I be using a simpler, DC signal to trace?

Guineafowl21:
Anyone? Bueller? Anyone?

The signal from the hybrid to the flash ADC is at -5.4V, when it should be -0.5V. If I bias it to the correct level with a suitable resistor, a trace appears on the screen.

However, something’s now happened and the scope won’t trigger in analogue mode. What might I have done?

HighVoltage:

--- Quote from: Guineafowl21 on July 01, 2019, 01:45:42 pm ---
From there, it reaches an upright, brown-coated hybrid chip pins 1 and 2. The output is on pin 3. The wave is still there, but looks distorted. I’m not sure if this is my measurement technique, though.


--- End quote ---
Most likely your "brown coated hybrid chip" is the culprit.
Show us some pictures

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